1. Field of the Invention
The present invention relates to an impedance conversion circuit which can attain a stable and superior transfer characteristic between input and output by using FETs (field-effect transistors).
2. Prior Art
When an input signal is to be transferred with the impedance converted, it is desirable to use an amplifier such as a source-follower circuit using FET(s).
However, as to the source-follower circuit using FET(s), the output voltage is not equal to the input voltage, since the output voltage depends on the threshold voltage (V.sub.T) of the FET used in the source-follower circuit. That is, in the source-follower circuit, the output voltage V.sub.out is given as: EQU V.sub.out .apprxeq.V.sub.in -V.sub.T
wherein, V.sub.in is the input voltage to the source-follower circuit. This means that there is a certain difference (V.sub.T) between the output voltage and the input voltage, further there is a defect in that the transfer characteristic between the input and the output voltage is unstable or fluctuates since the threshold voltage V.sub.T depends on the structure and production process of FET(s) or depends on temperature. Above all, the threshold voltage V.sub.T of a MOS.multidot.FET, which is typical for an insulated gate FET, depends largely on the production process, therefore, for the purpose of making the input/output characteristic of the source-follower circuit stable and constant the production process must be controlled accurately. Thus, obtaining stable and even input/output characteristics is very difficult.
The prior art will now be explained in greater detail as follows:
An exemplary conventional impedance conversion circuit is shown in FIG. 1. The circuit of FIG. 1 is a widely used fundamental source-follower circuit usually using a MOS.multidot.FET as the FET.
The operation of the circuit of FIG. 1, wherein the MOS.multidot.FET is used as FET 1, is explained as follows:
Provided that V.sub.T is the threshold voltage of MOS.multidot.FET 1, V.sub.in is the input voltage, V.sub.out is the output voltage, and the output impedance of MOS.multidot.FET 1 is sufficiently smaller than the impedance of the load resistance 2, then the output voltage V.sub.out is generally given as: EQU V.sub.out .apprxeq.V.sub.in -V.sub.T ( 1)
However, when the circuit in FIG. 1 is integrated in a semiconductor substrate, it is known that the threshold voltage V.sub.T varies on account of, what is called, the effect of substrate bias (body effect). Therefore, the threshold voltage is expressed as (V.sub.T +.DELTA.V.sub.T). Accordingly, the output voltage V.sub.out is given as: EQU V.sub.out =V.sub.in -(V.sub.T +.DELTA.V.sub.T) (2),
wherein, .DELTA.V.sub.T is the variation of the threshold voltage V.sub.T. Referring to the formula, it is obvious that the output voltage V.sub.out of the of FIG. 1 decreases by the amount of the sum of V.sub.T and .DELTA.V.sub.T from V.sub.in. The variation .DELTA.V.sub.T depends on the output voltage V.sub.out itself. Accordingly, the variation is not constant.
In addition, when using a MOS.multidot.FET, the threshold level voltage V.sub.T and the variation .DELTA.V.sub.T varies largely depending on the production process used and etc. Therefore, the stability of the threshold voltage V.sub.T and variation .DELTA.V.sub.T requires a high level of control over the production process. Further, V.sub.T and .DELTA.V.sub.T depend on temperature and accordingly to make V.sub.T and .DELTA.V.sub.T constant in the actual circuit is very difficult. Therefore, the circuit shown in FIG. 1 is extremely inappropriate for the purpose of realizing a stable and superior transfer characteristic.